Robots may be thought of in a variety of different ways. For purposes of this disclosure, it is useful to think of mobile robots as being statically stable, e.g., stable when un-powered, and dynamically stable, e.g., stable only when powered. Statically stable robots generally have at least three wheels. A significant but frequently overlooked problem is that statically stable wheeled mobile robots can easily become unstable. If the center of gravity is too high, or the robot accelerates/decelerates too rapidly, or is on a sloping surface, or is pushed too hard, the robot can tip over. Thus, statically stable wheeled robots tend to have low centers of gravity and large bases. However, short, wheeled robots having large bases do not interact well with humans and have trouble maneuvering in environments which are crowded or cluttered.
Work on dynamically stable wheeled robots has focused on two-wheeled devices. A two-wheeled robot with inverse pendulum control developed in Japan was demonstrated in 1994. (Y. S. Ha and S. Yuta., “Trajectory tracking control for navigation of self-contained mobile inverse pendulum,” In Proc. IEEE/RSJ Int'l. Conf. on Intelligent Robots and Systems, pages 1875-1882, 1994). The two-wheeled design eliminated the need for a third castoring wheel. The same group introduced a one-wheel balancing robot. (R. Nakajima, T. Tsubouchi, S. Yuta, and E. Koyanagi, “A development of a new mechanism of an autonomous unicycle,” In Proc. IEEE/RSJ Int'l. Conf. on Intelligent Robots and Systems, pages 906-12, Grenoble, France, Sep. 7-11, 1997). The wheel is a prolate ellipsoid like a rugby ball and is driven with an axle along the major axis. The body of the robot has a hinge above and perpendicular to this axis. The robot balances in the forward/backward directions by application of wheel torque in the manner of the two-wheeled design, and balances from side to side by leaning the body left or right at the actuated hinge. Recently, balancing wheelchairs and balancing 2-wheel Segway personal mobility devices have been introduced. The 2-wheel RMP robotic platforms based on the Segway are the subject of much recent development in robotic locomotion. (H. G. Nguyen, J. Morrell, K. Mullens, A. Burmeister, S. Miles, N. Farrington, K. Thomas, and D. Gage “Segway robotic mobility platform,” In SPIE Proc. 5609: Mobile Robots XVII, Philadelphia, Pa., October 2004).
The previous rolling/balancing machines cannot immediately drive in a given direction without first re-orienting the drive mechanism. For example, a two-wheel balancing machine such as the Segway RMP cannot maneuver in tight spaces by moving sideways; a robot based on such a machine could not open and close a door without knowing the precise location of the hinges to establish the correct turning radius. The rugby-ball robot cannot turn in place but can only turn in a wide arc.
Other work involves robots which can overcome those problems by using a single spherical wheel. One example is described in Endo, et al., “An Omnidirectional Vehicle on a Basketball” 0-7803-9177-2/05 IEEE. In such devices, the user must provide input in the form of leaning in the direction of motion. The greater the amount of the lean, the faster the device will travel. Another example is described in Havase, “ERROShpere: an Equilibrator Robot,” 0-7803-9137-3/05 IEEE. This robot, however, is believed to be capable of operation for only short periods of time before becoming unstable presumably due to imperfect sensing or and/or a faulty control method. In both of those cases, a single sphere is used as the contact between the device and the ground. In the case of the article by Endo, the device was a vehicle which could be ridden by a user. In the case of the article by Havase, the robot was constructed of a cage surrounding the sphere.
For certain tasks, a robot must be tall enough to interact with people at a reasonable height. Also, the robot must be skinny enough to easily make its way around without bumping into things or people. Thus, a need exists for a tall slender robot that is safe, agile, and can easily maneuver in cluttered and/or peopled environments and which can readily yield when pushed while remaining stable at all times.